Numerous minerals are present in subsurface earth formations in very small quantities which make their recovery extremely difficult. However, in most instances, these minerals are also extremely valuable, thereby justifying efforts to recover the same. An example of one such mineral is uranium. However, numerous other valuable minerals, such as copper, nickel, molybdenum, rhenium, silver, selenium, vanadium, thorium, gold, rare earth metals, etc., are also present in small quantities in some subsurface formations, alone and quite often associated with uranium. Consequently, the recovery of such minerals is fraught with essentially the same problems as the recovery of uranium and, in general, the same techniques for recovering uranium can also be utilized to recover such other mineral values, whether associated with uranium or occurring alone. Therefore, a discussion of the recovery of uranium will be appropriate for all such minerals.
Uranium occurs in a wide variety of subterranean strata such as granites and granitic deposits, pegmatites and pegmatite dikes and veins, and sedimentary strata such as sandstones, unconsolidated sands, limestones, etc. However, very few subterranean deposits have a high concentration of uranium. For example, most uranium-containing deposits contain from about 0.01 to 1 weight percent uranium, expressed as U.sub.3 O.sub.8 as is conventional practice in the art. Few ores contain more than about 1 percent uranium and deposits containing below about 0.1 percent uranium are considered so poor as to be currently uneconomical to recover unless other mineral values, such as vanadium, gold and the like, can be simultaneously recovered.
There are several known techniques for extracting uranium values from uranium-containing materials. One common technique is roasting of the ore, usually in the presence of a combustion supporting gas, such as air or oxygen, and recovering the uranium from the resultant ash. However, the present invention is directed to the extraction of uranium values by the utilization of aqueous leaching solutions. There are two common leaching techniques for recovering uranium values, which depend primarily upon the accessibility and size of the subterranean deposit. To the extent that the deposit containing the uranium is accessible by conventional mining means and is of sufficient size to economically justify conventional mining, the ore is mined, ground to increase the contact area between the uranium values in the ore and the leach solution, usually less than about 14 mesh but in some cases, such as limestones, to nominally less than 325 mesh, and contacted with an aqueous leach solution for a time sufficient to obtain maximum extraction of the uranium values. On the other hand, where the uranium-containing deposit is inaccessible or is too small to justify conventional mining, the aqueous leach solution is injected into the subsurface formation through at least one injection well penetrating the deposit, maintained in contact with the uranium-containing deposit for a time sufficient to extract the uranium values and the leach solution containing the uranium, usually referred to as a "pregnant" solution, is produced through at least one production well penetrating the deposit. It is the latter in-situ leaching of subsurface formations to which the present invention is directed.
The most common aqueous leach solutions are either aqueous acidic solutions, such as sulfuric acid solutions, or aqueous alkaline solutions, such as sodium carbonate and/or bicarbonate.
Aqueous acidic solutions are normally quite effective in the extraction of uranium values. However, as detailed hereinafter, aqueous acidic solutions generally cannot be utilized to extract uranium values from ore or in-situ from deposits containing high concentrations of acid-consuming gangue, such as limestone. While some uranium in its hexavalent state is present in ores and subterranean deposits, the vast majority of the uranium is present in its valence states lower than the hexavalent state. For example, uranium minerals are generally present in the form of uraninite, a natural oxide of uranium in a variety of forms such as UO.sub.2, UO.sub.3, UO.U.sub.2 O.sub.3 and mixed U.sub.3 O.sub.8 (UO.sub.2.2UO.sub.3), the most prevalent variety of which is pitchblende containing about 55 to 75 percent of uranium as UO.sub.2 and up to about 30 percent uranium as UO.sub.3. Other forms in which uranium minerals are found include coffinite, carnotite, a hydrated vanadate of uranium and potassium having the formula K.sub.2 (UO.sub.2).sub.2 (VO.sub.4).sub.2.3H.sub.2 O, and uranites which are mineral phosphates of uranium with copper or calcium, for example, uranite lime having the general formula CaO.2UO.sub.3.P.sub.2 O.sub.5.8H.sub.2 O. Consequently, in order to extract uranium values from subsurface formations with aqueous acidic leach solutions, it is necessary to oxidize the lower valence states of uranium to the soluble, hexavalent state.
Combinations of acids and oxidants which have been suggested by the prior art include nitric acid, hydrochloric acid or sulfuric acid, particularly sulfuric acid, in combination with air, oxygen, sodium chlorate, potassium permanganate, hydrogen peroxide and magnesium perchlorate and dioxide, as oxidants. However, the present invention is directed to the use of sulfuric acid leach solutions containing appropriate oxidants and other additives, such as catalysts.
In addition to the previously mentioned value of in-situ leaching of mineral values, where conventional mining of the ore is impossible or impractical, such leaching has numerous additional advantages. In-situ leaching eliminates the need for handling large tonnages of material, requires a minimum of surface installations and eliminates the need for disposing of final waste products, the last of which is particularly advantageous in the leaching of uranium. In addition, in more populated areas, in-situ leaching eliminates possible objections to undesirable open pits or structures. However, in-situ leaching is not without problems. Certain criteria must be met before an ore body may be considered suitable for in-situ leaching. Of particular importance are the characteristics of the surrounding strata. The ore should preferably be underlain by nonporous rock and should not be surrounded by badly fractured or channelled structures, any of which may lead to serious losses of leaching solution. Cement grouting or the use of special plastics or gels have been proposed as a means of sealing off possible areas of leakage. In addition, solution losses may be controlled to a certain extent by proper placement and usage of inlet and outlet wells. Such placement of injection and production wells may be any of the patterns commonly utilized in enhanced recovery of oil from subsurface earth formations. For example, there are the usual "five-spot" patterns in which four production wells are located at the corners of a square area and a single injection well is located in the center of the square. Other similar patterns are also known. A particularly useful pattern for the recovery of mineral values is one in which the injection wells are located at the corners of a hexagonal area and a single larger production well is located in the center. Techniques for completing the wells, i.e., casing, cementing and perforating, etc., locating the wells, controlling the flow of fluids through the formation, preventing loss of fluid to thief formations, improving areal sweep, etc. are well known to those skilled in the art of in-situ recovery of mineral values and particularly to those skilled in the art of enhanced oil recovery and therefore, the details of such techniques need not be set forth herein.
In addition to the previously mentioned problems of injection, flow through and production from a subsurface formation, additional problems in the in-situ recovery of mineral values from subsurface formations result from the character of the mineral-containing formations themselves. This is particularly true when sulfuric acid leach solutions are utilized.
Certain gangue constituents and other minerals present in mineral-containing formations often have more influence over the process selection than do other factors. Such gangue materials or minerals include calcium carbonate, usually present as calcite or limestone formations, calcium, magnesium carbonate originating in dolomite formations and certain clays, such as montmorillonite clay, magnesium carbonate present as magnesite, ferric carbonate (usually occurring as a mixture of ferric carbonate, ferric hydroxide and ferrous hydroxide), ferrous and ferric sulfides and free iron, the iron compounds generally occurring in most types of subsurface formations in varying quantities. Among the problems resulting from the presence of these gangue or mineral materials are excessive consumption of leach chemicals, substantial increases in the time required to recover the mineral values, plugging of the subsurface formation by the formation of insoluble precipitates, particularly when utilizing sulfuric acid leach solutions, utilization of a significant portion of the capacity of ion exchange materials (when such materials are utilized for the recovery of mineral values from leach solutions), plugging of ion exchange agents (where solid ion exchange agents are utilized) and generally a detrimental effect on the exchange capacity of ion exchange agents and a slowing down of the ion exchange processes and other obvious problems. Since most of these problems result from the precipitation of these materials in aqueous solutions and the present invention is directed in one primary aspect to the prevention of such precipitation, these materials will be referred to herein as "precipitate-forming cations" or "cations which form precipitates with sulfuric acid".
The most troublesome precipitate-forming cation is calcium. The calcium usually in the form of calcium carbonate, calcium, magnesium carbonate etc. will consume acid from an acidic leach solution directly in a ratio of about one pound of sulfuric acid per pound of calcium carbonate that may be present in the subsurface formation. It is generally considered that calcium in amounts of about ten to fifteen percent can be tolerated by acidic leach solutions but if more than fifteen percent calcium carbonate is present, acid cost would be prohibitive. In addition to consuming large quantities of acid, calcium carbonate also results in the previously mentioned problem of precipitation and plugging of a subsurface earth formation during in-situ recovery. This is due to the fact that the reaction of sulfuric acid on calcium carbonate is to form calcium sulfate which has an extremely low solubility in water. Calicum sulfate is soluble in water up to about 2 grams per liter or 0.2% by weight of water, more specifically, less than 1.6 grams per liter or 0.16% by weight of water. Consequently, once a sulfuric acid leach solution contains this amount of calcium sulfate, any further reaction of the sulfuric acid with the calcium carbonate to form calcium sulfate results in the formation of solid precipates which tend to plug the formation and result in reducing the exchange capacity of ion exchange agents and the plugging of solid ion exchange agents. This is further complicated by the fact that, when leach solutions are normally flowed through the subsurface earth formation, the leach solution containing the mineral values is treated at the surface of the earth to remove the mineral values from the leach solution and the leach solution is thereafter recycled one or more times through the formation to obtain optimum mineral value recovery. Accordingly, if the subsurface formation contains substantial amounts of calcium carbonate, the leach solution becomes saturated with calcium sulfate on the first pass through the formation and, therefore, during the second or subsequent passes through the formation, little further reaction of the sulfuric acid with the calcium carbonate is needed to cause the precipitation of calcium sulfate. Therefore, the only known technique in the prior art designed to overcome this problem, in the in-situ leaching of subsurface formations with sulfuric acid leach solutions, is to start with a sulfuric acid solution containing 1.0 to 1.5 grams of sulfuric acid per liter of leach solution or about 0.1 to 0.15 weight percent sulfuric acid in the leach solution. This leach solution is then circulated through the subsurface formation until all of the calcium carbonate has been reacted or neutralized, usually indicated by breakthrough or detection of acid in the leach solution produced from the producing well. Thereafter the concentration of acid in the leach solution is increased, for example, up to about 5 grams per liter or 0.5 weight percent. The obvious disadvantages of this technique include the large consumption of acid, as well as the increase in time necessary to carry out the process.